1. Field of the Invention
The present invention relates to planarization method and apparatus for patterns on a wafer surface with polishing used in manufacturing process for semiconductor integrated circuit. The present invention particularly relates to a wafer holder that provides high process uniformity and high reliability over an entire surface area including a wafer outer periphery.
2. Description of the Related Art
Recently, importance of planarization of wafer surface with semiconductor devices formed thereon have been increasing because of a problem on insufficient focusing margin for exposure optical system in lithography process due to increasing density and reducing size of semiconductor device. One of the wafer planarization techniques is a polishing technique so-called xe2x80x9cChemical Mechanical Polishing (CMP)xe2x80x9d shown in FIG. 3.
A polishing pad 16 is attached on a rotation platen 15 and rotated. The polishing pad 16 is, for example, made by slicing polyurethane foam resin into form of thin sheet and molding it. Different materials form of thin sheet and molding it. Different materials and surface fine structures for the polishing pad 16 are selected depending on the type of workpiece and desired roughness of the surface to be finished. On the other hand, as described in Japanese Patent Laid-Open Publication No. 11-198025, a retainer 18 is provided for a wafer 2 to be processed, which is intended to prevent the wafer from projecting by horizontal force due to friction between the wafer and the polishing pad, and the wafer is pressed against the polishing pad 16 with constant pressure. Projections of insulator film on the wafer surface are polished and removed for substantial planarization, by pressing the backside of the wafer 2 with flexible means such as air or sponge with rotation of a wafer holder 17 to press the wafer against the polishing pad 16, and supplying a polishing slurry 14 on the polishing pad 16.
When insulator film like silicon dioxide is polished, colloidal silica is typically used as polishing slurry. The colloidal silica is prepared by suspending fine silica particle about 30 nm diameter in alkali aqueous solution such as aqueous potassium hydroxide. It is characterized in that it provides much higher process efficiency and smooth surface with less process damage as compared with mechanical polishing with only abrasive grain since it additionally has alkali chemical reaction effect.
As described above, the method in which workpiece is processed while polishing slurry is supplied between a polishing pad and the workpiece, is well known as xe2x80x9cloose abrasive grain polishing techniquexe2x80x9d. However, it has a problem for pattern size dependency that adequate planarization may be not provided depending on the pattern type and height profile condition, a problem of extremely high cost for consumable supplies such as polishing slurry and polishing pads, and subject issue like inadequate long-term stability due to polishing pad consumption.
A planarization concept with bounded abrasive grain polishing is disclosed in PCT/JP95/01814 for eliminating such disadvantage of planalization with loose abrasive grain polishing.
The new planalization technique is characterized in that in the polishing apparatus shown in FIG. 3 it uses a special polishing wheel 1 that is hardness-controlled in best optimization, instead of conventional polishing pads. Specifically, the modulus of elasticity of the polishing wheel 1 is 5 to 500 kg/mm2, one tenth to one hundredth of the modulus of elasticity of conventional typical polishing wheel while it is five to fifty times as hard polishing pad such as pads of hard polyurethane foam.
The type of the slurry is preferably such as silicon dioxide, cerium oxide, or aluminum oxide. The slurry with 0.01 to 1 xcexcm in grain diameter provides good process efficiency without scratch occurrence. The resin for binding these abrasive grains is preferably high purity organic based resin such as phenol based resin or polyethylene based resin. The abrasive grain is kneaded in binding resin, then the resin is solidified in adequate pressure and, if necessary, is subject to treatment such as thermal curing. In the preparation method, the hardness of polishing wheel to be formed can be controlled by the type of binding resin and the pressing pressure, and is set at 5 to 500 kg/mm2 in this technique.
When pure water is supplied as polishing solution to a polishing wheel that is formed by binding cerium oxide abrasive grain with 1 xcexcm in grain size with phenol based resin or polyethylene based resin such that the modulus of elasticity of the polishing wheel is 100 kg/mm2, then silicon dioxide film with 1 xcexcm in thickness is processed with the solution, very good planarization performance is provided such that no scratch occurs and processing rate is within 0.3xc2x10.011 xcexcm/minute for all type of patterns raging from 10 mm to 0.5 xcexcm in pattern width. It is verified by the inventor that the scratch free process could not be compatible with the good planarization performance without the bounded abrasive grain process utilizing the polishing wheel with optimized modulus of elasticity.
As described above, planarization techniques that use a polishing wheel as a polishing tool have many advantages. On the other hand, they have a disadvantage for process uniformity because of much higher modulus elasticity of the polishing wheel than polishing pads.
At the time of performing loose abrasive grain polishing with a polishing pad, it is done with the retainer 18 being pressed against the polishing pad, as described above with reference to FIG. 3. It causes the retainer 18 to be worn with the wafer polishing. Balancing between pressing pressure applied to the wafer backside during polishing and receiving pressure at the wafer front-side is made with help of elastic deformation of the flexible polishing pad. However, when the retainer 18 is worn, it needs to be replaced since the pressure distribution over a wafer surface is no longer uniform. In the case of bounded abrasive grain polishing utilizing a polishing wheel with high modulus of elasticity, it is more difficult to continuously provide good uniformity than CMP since there is almost no deformation effect of the polishing wheel itself.
In the case of bounded abrasive grain polishing using a polishing wheel with high modulus of elasticity, there is a problem that since friction occurring during process is one time and half to two times higher than friction in loose abrasive grain polishing using a polishing pad the periphery of wafer 2 tends to be over-polished because of processed wafer 2 being pressed against the retainer 18, making it difficult to narrow edge exclusion that is exclusion area at the wafer periphery.
As described above, bounded abrasive grain polishing using a polishing wheel has a disadvantage that uniformity is inadequate because of inadequate deformation absorbing capability of the polishing wheel in conventional wafer holders, a disadvantage that edge exclusion cannot be narrowed, and so on.
An object of the present invention is to provide a process apparatus including a wafer holder, and a process method, in which high planarization performance, scratch free process, narrow edge exclusion and high uniformity can be maintained for more than 10,000 processed wafers.
The object can be achieved by providing means for keeping a retainer ring and surface of a polishing wheel non-contact with each other and controlling the gap therebetween within a certain range and by setting compression strength of the retainer ring at more than 3,000 kg/cm2.